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The relationship between excluded mineral matter and the abrasion index of a coal
Fuel 83 (2004) 359–364 www.fuelfirst.com
The relationship between excluded mineral matter and the abrasion index of a coalq J.J. Wellsa,1, F. Wigleya, D.J. Fosterb, W.H. Gibbc, J. Williamsona,* a
Department of Materials, Imperial College London, London SW7 2BP, UK b Mitsui Babcock plc, Renfrew, Scotland PA4 8DJ, UK c Powergen UK plc, Power Technology Centre, Ratcliffe on Soar, Nottingham NG11 OEE, UK Received 30 October 2002; revised 22 July 2003; accepted 29 July 2003; available online 23 August 2003
Abstract Predictions of the wear rates of components in grinding mills at pulverised coal-fired power stations are currently made using empirical relationships based on the ash content of the coals. However, modern coal characterisation techniques now allow the mineral inclusions in a coal that are responsible for the abrasive nature of the coal to be accurately characterised. Hence, there is scope to make improved predictions of wear based on a detailed knowledge of the mineral matter in a particular coal. It is first necessary, however, to understand the nature of the minerals and properties of the minerals in a coal that would contribute to abrasive wear. In this study known quantities of quartz, pyrite and slate have been added to a washed coal and the Abrasion Indices of the coal/mineral mixtures have been measured. The results show how the size, shape and hardness of excluded mineral matter contribute to the abrasive properties of a coal. q 2004 Elsevier Ltd. All rights reserved. Keywords: Coal minerals; Coal milling; Abrasion; Erosion
1. Introduction Wear and abrasion of mills used to grind coals for pulverised coal-fired boilers can result in an increase in the particle size distribution of the ground coals, and this in turn can affect the levels of unburnt carbon in the ash and give problems with the electrostatic precipitators, cause instability of the combustion process and give reduced combustion efficiency leading to problems controlling NOx levels from low NOx burners. In addition, wear of grinding elements can lead to ball and ring breakages. To combat wear, planned maintenance programmes are scheduled for mills and associated pipe work based on predictions of the rate at which mill components will wear with a particular coal. q This paper was presented at the Fourth UK Meeting on Coal Research and its Applications organised by the Coal Research Forum, September 2002. * Corresponding author. Tel.: þ44-171-594-6747; fax: þ 44-171-5946748. E-mail address: [email protected] (J. Williamson). 1 Present address: Innogy plc, Windmill Hill Business Park, Swindon, Wiltshire SN5 6PB, UK.
0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00262-X
The predictions of wear are currently based on empirical relationships, as described by Raask [1]. Coals contain a wide range of minerals, but it is generally acknowledged that quartz and pyrite, minerals that are harder than steel, are the main components in the coal responsible for the wear and abrasion. Clays, carbonates, sulphates and phosphate minerals are much softer and have little effect on wear processes. The nature of the quartz and pyrite minerals in terms of size, shape and degree of inclusion can vary considerably from one coal to another. Only those minerals that are released from the coal during the grinding process, mineral matter that is termed ‘excluded’, will cause abrasive damage. Minerals that remain within the coal particles, known as ‘included’ mineral matter, will not be abrasive, nor will the carbonaceous material which acts as a lubricant during the grinding process. It has previously been suggested that quartz is 2– 5 times more abrasive than pyrite on a wt% basis in coal. This factor was attributed to quartz which is generally found as large ‘excluded’ particles, whereas the pyrite is often ‘included’ in the soft clays and the coal matrix [1]. However, this has been determined empirically and was based on studies of
360
J.J. Wells et al. / Fuel 83 (2004) 359–364
UK coals, and has not been found to be applicable to world traded coals. Abrasive wear has been linked to variables such as particle size [2] and shape [3]. Given that the mineral matter in coal can now be characterised in terms of the proportions of included and excluded minerals using a Scanning Electron Microscopy (SEM), there is the scope for developing improved predictions for the abrasiveness of a coal. This paper presents the results of a study in which the Abrasion Index has been measured as quartz, pyrite and shale have been added singly and in combination to a washed Rossington coal (5 wt% ash). The data is therefore applicable to the effects of the excluded mineral matter on abrasion wear, including the relative contributions to wear made by the quartz and pyrite. Thus mineral characterisations of a raw coal, and of a ground coal that incorporates the amounts of included and excluded minerals, may be used to improve predictions of mill wear.
2. Experimental Samples of angular quartz and pyrite were obtained as large single crystals and then crushed down and sieved to provide various size ranges. The rounded quartz was obtained as soft sand and sieved to a single size fraction. Slate was used as a substitute for shale and was crushed and sieved to give the desired size range. Details of the different minerals used for the abrasion tests are given in Table 1. Backscattered SEM images of the different samples are shown in Fig. 1. The size ranges were selected as being typical of grain sizes for quartz, pyrite, and shales found in raw coals. The angular quartz and pyrite were pure, being derived from large single crystals. The rounded quartz included a few inclusions of feldspar (rounded, brighter particles in Fig. 1(d)), that have a similar hardness to quartz. The slate contained , 15% of fine quartz particles and , 10% of iron oxide, both of which have a hardness of 7 on the Mohs scale. There was also , 5 wt% of apatite in the slate, which has a hardness of 5, similar to the hardness of mild steel. The size distribution of each mineral addition was determined using a Malvern particle size analyser. Table 1 Size and shape of the different mineral additions Mineral addition
Grain size (mm)
Angular quartz
,50 50– 100 100– 212 100– 212 ,50 50– 100 100– 212
Rounded quartz Angular pyrite Slate
Fig. 1. Backscattered SEM images of the different mineral additions: (a) angular quartz ,50 mm, (b) angular quartz 50–100 mm, (c) angular quartz 100 – 212 mm, (d) rounded quartz 100 – 212 mm, (e) angular pyrite ,50 mm, (f) angular pyrite 50–100 mm, and (g) slate 100–212 mm.
The Abrasion Index was calculated according to British Standard BS 1016-111:1998 using a Yancey, Geer and Price (YGP) test rig. The coal used for the tests was a washed Rossington coal with a dry ash content of 5.1 wt%. The bulk chemical ash analysis of this coal is given in Table 2. A CCSEM characterisation of the mineral matter in the coal revealed pyrite and clays as the only minerals present in significant amounts. The pyrite content was found to be 0.2 vol% of the coal, while the amount of quartz was negligible. For the Abrasion Index test the coal was air-dried coal and then crushed to give a particle size of less than
J.J. Wells et al. / Fuel 83 (2004) 359–364
361
Table 2 Ash content and ash analysis of the Rossington coal (wt%) Ash content
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
TiO2
P2O5
SO3
5.1
39.7
26.5
16.0
5.3
1.6
4.2
1.9
1.2
0.2
3.3
6.7 mm, with a maximum of 30 wt% fines. Coning and quartering of the coal produced four similar batches for mineral additions, including a sample without mineral additions for providing a standard Abrasion Index base line. Additions of 1, 3 and 5 wt% of each mineral were made to a 2 kg sample of the coal. Several of the tests were repeated to test the repeatability of the Abrasion Index test. The YGP test rig consists of four mild steel blades that rotate in an enclosed container of specified dimensions. The blades are carefully positioned to give an accurate distance between the tip of the blades and the sidewalls of the vessel. A 2 kg batch of coal is placed in the YGP test rig and the blades are then rotated at a specified velocity for 12,000 ^ 16 revolutions. The Abrasion Index is given by the total weight lost of the four carbon steel blades, expressed as milligrams of metal lost per kilogram of coal used for the test.
the reasons are discussed when considering the effects of grain size on abrasion. 3.2. The effect of grain size on the abrasiveness of the coal/mineral mixture Fig. 3(a) shows a graph of Abrasion Index for the coal/ mineral mixtures versus wt% addition for three different size ranges of angular quartz. The Abrasion Index was observed to increase with quartz size, however, the increase from the 50 – 100 mm size to the 100 – 212 mm size fraction is very small. Fig. 3(b) shows a graph of the Abrasion Index per wt% addition versus average grain size. The data has been taken from the results obtained for a 3 wt% addition. Values for the Abrasion Index at 0% mineral addition (, 10 mg/kg for each type of quartz), representing the abrasive wear of the undoped coal have been taken from the Abrasion Index values for the 3 wt% addition to show the effect of the excluded mineral additions. Fig. 3(b) shows that above
3. Results and discussion 3.1. The effect of angular quartz and angular pyrite on the abrasiveness of the coal/mineral mixture Fig. 2(a) shows how the Abrasion Index of the coal/ mineral mixtures varied as a function of the wt% addition of quartz and pyrite in the , 50 mm, and 50– 100 size ranges. If the quartz and pyrite are considered on a wt% addition basis, the quartz is seen to be effectively twice as abrasive as pyrite. This is similar to the effect mentioned by Raask [1], except that in the current work all the mineral additions were excluded from the coal particles. Abrasive wear is a function of the number of particles scoring the surface of material being abraded. Pyrite is twice as dense as quartz, and hence for a given mass of particles with a fixed grain size there would be twice as many quartz particles present as pyrite. Fig. 2(b) shows the same data, but this time the Abrasion Index has been plotted against the vol% addition of added minerals. The abrasiveness of the quartz and pyrite is now seen to be the same when considering a similar grain size. Hence any model used to predict the abrasiveness of a coal as a function of the mineral matter content should take into account the relative densities of the minerals concerned. The Abrasion Index results were found to be very reproducible. The tests were not repeated a sufficient number of times for a statistical analysis, since each test required 2 kg of coal and , 1.5 man-hours. The non-linear relationships seen in Fig. 2 are believed to be real, and
Fig. 2. Abrasion Index of the coal/mineral mixtures as a function of mineral addition for two different size ranges of quartz and pyrite, (a) by wt% of mineral addition, and (b) by vol% of mineral addition.
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Fig. 4. Comparison of the Abrasion Index versus wt% of mineral addition for the angular and rounded quartz particles in the 100–212 mm size range.
Fig. 3. (a) The Abrasion Index versus wt% mineral addition for the three different size ranges of angular quartz, and (b) the Abrasion Index per wt% addition versus average grain size showing the abrasion size effect.
a critical grain size the abrasive nature of the coal/mineral addition mixture becomes independent of the grain size for the angular quartz. Similar size-effects have been observed in other systems, and the critical size has been reported to be , 100 mm [2]. For each series of additions, the more mineral of a given size range added the higher the value of the Abrasion Index. However, this is not a linear relationship. As more of a mineral at a given size is added, the incremental increases in abrasiveness become less. This effect has also previously been noted [2], and suggests that as more of an abradant of a given particle size is added the effective particle size of that abradant is considered to get smaller. This means that there would be a critical amount for each mineral addition above which no further change in abrasiveness would be observed. This effect has been shown to be very pronounced for rounded particles, but insignificant for angular particles [2]. 3.3. The effect of particle shape on the abrasiveness of the coal/mineral mixture Fig. 4 compares the values of the Abrasion Index for the coal/quartz mixtures with rounded and angular quartz additions in the 100 –212 mm size range. The rounded particles seem to be far less abrasive than the angular particles. This is understandable since rounded particles are
likely to plastically deform a metal surface, whereas angular particles are more likely to cut into and remove metal from the surface. For predictive purposes it would be useful to put a measure on the angularity of the particles, but in practise this is difficult. An approach suggested by Stachowiak [4] uses a technique to generate a ‘spike parameter-quadratic fit based on curve fitting to major boundary features’. However, in terms of raw coal, this would require the image analysis of hundreds of pieces of coal and would be an impractical measure to attempt. The non-linear relationship between values of the Abrasion Index and amount of mineral added is very pronounced for the rounded quartz. In fact, above 5 wt% of rounded quartz, it appears as though the Abrasion Index would be independent of further additions. This is not the case for the angular quartz, which shows a relationship much closer to linearity. It seems therefore, that a key factor in predicting the abrasiveness of a coal based on its mineral content may require an assessment of the angularity of the quartz and pyrite in the raw coal, together with an understanding the non-linear relationship between the amount of mineral present and abrasiveness as a function of particle shape. 3.4. The effect of adding rounded quartz and angular pyrite of different sizes to the abrasiveness of the coal/mineral mixture Fig. 5 shows the values for the Abrasion Index of the coal/mineral mixtures with additions of the rounded quartz in the 100– 212 mm bracket, angular pyrite in the 50– 100 mm bracket, and a mixture of 50% rounded quartz (100 –212 mm) and 50% angular pyrite (50 – 100 mm). The dotted line is the calculated amount of abrasion the mixture of quartz and pyrite would have given based on the measurements of their individual contributions. This graph shows that the overall abrasion from two different mineral additions is not simply an additive function of the abrasion for each component.
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363
This non-additive behaviour may be due to the rounded particles plastically deforming the metal surfaces, leading to locally work-hardened metal that could then reduce the cutting action of the angular particles.
the quartz and iron oxide to the coal without the rest of the slate (clays). The abrasion as a result of the other constituents in the slate makes up the difference between the two lines. The difference between the actual values for the abrasion with slate and the values calculated for abrasion from equivalent amounts of pure quartz and iron oxide in the slate is very small, especially considering the errors associated with determining the amounts of quartz and iron oxide in the slate. However, the slate does appear to have been more abrasive than the quartz and iron oxide contents alone would have suggested. Apatite, with a hardness similar to that of steel, is probably making a small contribution to the overall abrasiveness of the slate. It is impossible to determine if the aluminosilicate matrix is contributing to the abrasion or not, but if it is then it would be a very small contribution. It has been observed before that a softer material can abrade a hard material (e.g. the wear of diamond Atomic Force Microscopy tips on silicon wafers), but the effect is small [5]. In the case of a coal, any contribution from minerals below a hardness of 4.5 to the overall abrasiveness of the coal is likely to be negligible, except possibly for coals with very low quartz and pyrite contents.
3.5. The effect of slate on the abrasiveness of the coal/mineral mixture
4. Conclusions
Fig. 5. Abrasion Index versus vol% mineral addition for angular pyrite (50– 100 mm), rounded quartz (100–212 mm) and a 50:50 wt% mixture of the angular pyrite and rounded quartz. The dashed line represents the calculated Abrasion Index obtained by adding the individual abrasive values for the angular pyrite and rounded quartz for a 50:50 ratio.
The slate particles were found to be the least abrasive of the minerals added to the coal. This is as expected, given that the hardness of slates on the Mohs scale is , 3.5. However, there are components of the slate that are harder than steel, namely the quartz and iron oxides, which both have hardness values of 7. Effectively 15 wt% of the slate has a hardness greater than steel, and this material is generally in the 0 –50 mm size range. Fig. 6 shows a graph of the Abrasion Index for the coal/slate mixtures versus wt% addition of slate in the 100 –212 mm size range. The grey line shows the calculated Abrasion Index that might be expected for wear resulting from the equivalent of adding
Fig. 6. Abrasion Index versus wt% addition of the slate (100– 212 mm). The grey line is the calculated Abrasion Index for the cumulative effect of the quartz and iron oxide contents of the slate.
The effects of adding increasing amounts of different minerals to a washed Rossington coal on the Abrasion Index have been measured using a YGP test rig. Quartz and pyrite, with hardness values greater than that of mild steel, made significant contributions to the abrasiveness of the coal. Components of a slate with a lower hardness than the steel made very little contribution to the overall abrasiveness of the coal. Hence it is reasonable to assume that only those minerals in a coal that are harder than steel will significantly contribute to the abrasive nature of a coal. The abrasiveness of free angular quartz and pyrite was found to be similar when considered as a vol% addition. It is important to take into account the relative density of different minerals when considering abrasive wear. As the particle size of the quartz and pyrite increased up to a size of , 100 mm there was an increase in the value of the Abrasion Index for the coal/mineral mixtures. Above this size, values of the Abrasion Index appeared to be relatively independent of mineral size. This effect, known as the abrasion size effect, has been observed in other systems and is thought to be more significant for rounded, as opposed to angular, particles. Angular particles were far more abrasive than rounded particles. This is probably due to rounded particles causing plastic deformation of the metal surface, rather than cutting into the metal as an angular particle would do. The effect of adding both angular and rounded particles to the same batch of coal was not additive suggesting that the different abrasion mechanisms of angular and rounded particles are
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not mutually exclusive. Hence, for predictive purposes it may be necessary to determine the degree of angularity of hard minerals in a coal before grinding and milling.
also grateful to Powergen and Mitsui Babcock plc for their permission to publish this paper.
Acknowledgements
References
This work forms part of a DTI project funded under the ‘Cleaner Coal Research and Development’ programme (Project No. 218). The authors are grateful to Powergen for the use of their YGP Abrasion facilities and for preparing the coal according to British Standard BS 1016-111:1998, and to Mitsui Babcock plc for supplying the coal. The authors are
[1] Raask E. Mineral impurities in coal combustion: behaviour, problems, and remedial measures. New York: Hemisphere Publishing Corporation; 1985. p. 243 –8. [2] Ga˚hlin R, Jacobson S. Wear 1999;224:118– 25. [3] Stachowiak GB, Stachowiak GW. Wear 2001;249:201–7. [4] Stachowiak GW. Wear 2000;241:214–9. [5] Khurshudov AG, Kato K, Koide H. Wear 1997;203:22–7.
The relationship between excluded mineral matter and the abrasion index of a coalq J.J. Wellsa,1, F. Wigleya, D.J. Fosterb, W.H. Gibbc, J. Williamsona,* a
Department of Materials, Imperial College London, London SW7 2BP, UK b Mitsui Babcock plc, Renfrew, Scotland PA4 8DJ, UK c Powergen UK plc, Power Technology Centre, Ratcliffe on Soar, Nottingham NG11 OEE, UK Received 30 October 2002; revised 22 July 2003; accepted 29 July 2003; available online 23 August 2003
Abstract Predictions of the wear rates of components in grinding mills at pulverised coal-fired power stations are currently made using empirical relationships based on the ash content of the coals. However, modern coal characterisation techniques now allow the mineral inclusions in a coal that are responsible for the abrasive nature of the coal to be accurately characterised. Hence, there is scope to make improved predictions of wear based on a detailed knowledge of the mineral matter in a particular coal. It is first necessary, however, to understand the nature of the minerals and properties of the minerals in a coal that would contribute to abrasive wear. In this study known quantities of quartz, pyrite and slate have been added to a washed coal and the Abrasion Indices of the coal/mineral mixtures have been measured. The results show how the size, shape and hardness of excluded mineral matter contribute to the abrasive properties of a coal. q 2004 Elsevier Ltd. All rights reserved. Keywords: Coal minerals; Coal milling; Abrasion; Erosion
1. Introduction Wear and abrasion of mills used to grind coals for pulverised coal-fired boilers can result in an increase in the particle size distribution of the ground coals, and this in turn can affect the levels of unburnt carbon in the ash and give problems with the electrostatic precipitators, cause instability of the combustion process and give reduced combustion efficiency leading to problems controlling NOx levels from low NOx burners. In addition, wear of grinding elements can lead to ball and ring breakages. To combat wear, planned maintenance programmes are scheduled for mills and associated pipe work based on predictions of the rate at which mill components will wear with a particular coal. q This paper was presented at the Fourth UK Meeting on Coal Research and its Applications organised by the Coal Research Forum, September 2002. * Corresponding author. Tel.: þ44-171-594-6747; fax: þ 44-171-5946748. E-mail address: [email protected] (J. Williamson). 1 Present address: Innogy plc, Windmill Hill Business Park, Swindon, Wiltshire SN5 6PB, UK.
0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00262-X
The predictions of wear are currently based on empirical relationships, as described by Raask [1]. Coals contain a wide range of minerals, but it is generally acknowledged that quartz and pyrite, minerals that are harder than steel, are the main components in the coal responsible for the wear and abrasion. Clays, carbonates, sulphates and phosphate minerals are much softer and have little effect on wear processes. The nature of the quartz and pyrite minerals in terms of size, shape and degree of inclusion can vary considerably from one coal to another. Only those minerals that are released from the coal during the grinding process, mineral matter that is termed ‘excluded’, will cause abrasive damage. Minerals that remain within the coal particles, known as ‘included’ mineral matter, will not be abrasive, nor will the carbonaceous material which acts as a lubricant during the grinding process. It has previously been suggested that quartz is 2– 5 times more abrasive than pyrite on a wt% basis in coal. This factor was attributed to quartz which is generally found as large ‘excluded’ particles, whereas the pyrite is often ‘included’ in the soft clays and the coal matrix [1]. However, this has been determined empirically and was based on studies of
360
J.J. Wells et al. / Fuel 83 (2004) 359–364
UK coals, and has not been found to be applicable to world traded coals. Abrasive wear has been linked to variables such as particle size [2] and shape [3]. Given that the mineral matter in coal can now be characterised in terms of the proportions of included and excluded minerals using a Scanning Electron Microscopy (SEM), there is the scope for developing improved predictions for the abrasiveness of a coal. This paper presents the results of a study in which the Abrasion Index has been measured as quartz, pyrite and shale have been added singly and in combination to a washed Rossington coal (5 wt% ash). The data is therefore applicable to the effects of the excluded mineral matter on abrasion wear, including the relative contributions to wear made by the quartz and pyrite. Thus mineral characterisations of a raw coal, and of a ground coal that incorporates the amounts of included and excluded minerals, may be used to improve predictions of mill wear.
2. Experimental Samples of angular quartz and pyrite were obtained as large single crystals and then crushed down and sieved to provide various size ranges. The rounded quartz was obtained as soft sand and sieved to a single size fraction. Slate was used as a substitute for shale and was crushed and sieved to give the desired size range. Details of the different minerals used for the abrasion tests are given in Table 1. Backscattered SEM images of the different samples are shown in Fig. 1. The size ranges were selected as being typical of grain sizes for quartz, pyrite, and shales found in raw coals. The angular quartz and pyrite were pure, being derived from large single crystals. The rounded quartz included a few inclusions of feldspar (rounded, brighter particles in Fig. 1(d)), that have a similar hardness to quartz. The slate contained , 15% of fine quartz particles and , 10% of iron oxide, both of which have a hardness of 7 on the Mohs scale. There was also , 5 wt% of apatite in the slate, which has a hardness of 5, similar to the hardness of mild steel. The size distribution of each mineral addition was determined using a Malvern particle size analyser. Table 1 Size and shape of the different mineral additions Mineral addition
Grain size (mm)
Angular quartz
,50 50– 100 100– 212 100– 212 ,50 50– 100 100– 212
Rounded quartz Angular pyrite Slate
Fig. 1. Backscattered SEM images of the different mineral additions: (a) angular quartz ,50 mm, (b) angular quartz 50–100 mm, (c) angular quartz 100 – 212 mm, (d) rounded quartz 100 – 212 mm, (e) angular pyrite ,50 mm, (f) angular pyrite 50–100 mm, and (g) slate 100–212 mm.
The Abrasion Index was calculated according to British Standard BS 1016-111:1998 using a Yancey, Geer and Price (YGP) test rig. The coal used for the tests was a washed Rossington coal with a dry ash content of 5.1 wt%. The bulk chemical ash analysis of this coal is given in Table 2. A CCSEM characterisation of the mineral matter in the coal revealed pyrite and clays as the only minerals present in significant amounts. The pyrite content was found to be 0.2 vol% of the coal, while the amount of quartz was negligible. For the Abrasion Index test the coal was air-dried coal and then crushed to give a particle size of less than
J.J. Wells et al. / Fuel 83 (2004) 359–364
361
Table 2 Ash content and ash analysis of the Rossington coal (wt%) Ash content
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
TiO2
P2O5
SO3
5.1
39.7
26.5
16.0
5.3
1.6
4.2
1.9
1.2
0.2
3.3
6.7 mm, with a maximum of 30 wt% fines. Coning and quartering of the coal produced four similar batches for mineral additions, including a sample without mineral additions for providing a standard Abrasion Index base line. Additions of 1, 3 and 5 wt% of each mineral were made to a 2 kg sample of the coal. Several of the tests were repeated to test the repeatability of the Abrasion Index test. The YGP test rig consists of four mild steel blades that rotate in an enclosed container of specified dimensions. The blades are carefully positioned to give an accurate distance between the tip of the blades and the sidewalls of the vessel. A 2 kg batch of coal is placed in the YGP test rig and the blades are then rotated at a specified velocity for 12,000 ^ 16 revolutions. The Abrasion Index is given by the total weight lost of the four carbon steel blades, expressed as milligrams of metal lost per kilogram of coal used for the test.
the reasons are discussed when considering the effects of grain size on abrasion. 3.2. The effect of grain size on the abrasiveness of the coal/mineral mixture Fig. 3(a) shows a graph of Abrasion Index for the coal/ mineral mixtures versus wt% addition for three different size ranges of angular quartz. The Abrasion Index was observed to increase with quartz size, however, the increase from the 50 – 100 mm size to the 100 – 212 mm size fraction is very small. Fig. 3(b) shows a graph of the Abrasion Index per wt% addition versus average grain size. The data has been taken from the results obtained for a 3 wt% addition. Values for the Abrasion Index at 0% mineral addition (, 10 mg/kg for each type of quartz), representing the abrasive wear of the undoped coal have been taken from the Abrasion Index values for the 3 wt% addition to show the effect of the excluded mineral additions. Fig. 3(b) shows that above
3. Results and discussion 3.1. The effect of angular quartz and angular pyrite on the abrasiveness of the coal/mineral mixture Fig. 2(a) shows how the Abrasion Index of the coal/ mineral mixtures varied as a function of the wt% addition of quartz and pyrite in the , 50 mm, and 50– 100 size ranges. If the quartz and pyrite are considered on a wt% addition basis, the quartz is seen to be effectively twice as abrasive as pyrite. This is similar to the effect mentioned by Raask [1], except that in the current work all the mineral additions were excluded from the coal particles. Abrasive wear is a function of the number of particles scoring the surface of material being abraded. Pyrite is twice as dense as quartz, and hence for a given mass of particles with a fixed grain size there would be twice as many quartz particles present as pyrite. Fig. 2(b) shows the same data, but this time the Abrasion Index has been plotted against the vol% addition of added minerals. The abrasiveness of the quartz and pyrite is now seen to be the same when considering a similar grain size. Hence any model used to predict the abrasiveness of a coal as a function of the mineral matter content should take into account the relative densities of the minerals concerned. The Abrasion Index results were found to be very reproducible. The tests were not repeated a sufficient number of times for a statistical analysis, since each test required 2 kg of coal and , 1.5 man-hours. The non-linear relationships seen in Fig. 2 are believed to be real, and
Fig. 2. Abrasion Index of the coal/mineral mixtures as a function of mineral addition for two different size ranges of quartz and pyrite, (a) by wt% of mineral addition, and (b) by vol% of mineral addition.
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J.J. Wells et al. / Fuel 83 (2004) 359–364
Fig. 4. Comparison of the Abrasion Index versus wt% of mineral addition for the angular and rounded quartz particles in the 100–212 mm size range.
Fig. 3. (a) The Abrasion Index versus wt% mineral addition for the three different size ranges of angular quartz, and (b) the Abrasion Index per wt% addition versus average grain size showing the abrasion size effect.
a critical grain size the abrasive nature of the coal/mineral addition mixture becomes independent of the grain size for the angular quartz. Similar size-effects have been observed in other systems, and the critical size has been reported to be , 100 mm [2]. For each series of additions, the more mineral of a given size range added the higher the value of the Abrasion Index. However, this is not a linear relationship. As more of a mineral at a given size is added, the incremental increases in abrasiveness become less. This effect has also previously been noted [2], and suggests that as more of an abradant of a given particle size is added the effective particle size of that abradant is considered to get smaller. This means that there would be a critical amount for each mineral addition above which no further change in abrasiveness would be observed. This effect has been shown to be very pronounced for rounded particles, but insignificant for angular particles [2]. 3.3. The effect of particle shape on the abrasiveness of the coal/mineral mixture Fig. 4 compares the values of the Abrasion Index for the coal/quartz mixtures with rounded and angular quartz additions in the 100 –212 mm size range. The rounded particles seem to be far less abrasive than the angular particles. This is understandable since rounded particles are
likely to plastically deform a metal surface, whereas angular particles are more likely to cut into and remove metal from the surface. For predictive purposes it would be useful to put a measure on the angularity of the particles, but in practise this is difficult. An approach suggested by Stachowiak [4] uses a technique to generate a ‘spike parameter-quadratic fit based on curve fitting to major boundary features’. However, in terms of raw coal, this would require the image analysis of hundreds of pieces of coal and would be an impractical measure to attempt. The non-linear relationship between values of the Abrasion Index and amount of mineral added is very pronounced for the rounded quartz. In fact, above 5 wt% of rounded quartz, it appears as though the Abrasion Index would be independent of further additions. This is not the case for the angular quartz, which shows a relationship much closer to linearity. It seems therefore, that a key factor in predicting the abrasiveness of a coal based on its mineral content may require an assessment of the angularity of the quartz and pyrite in the raw coal, together with an understanding the non-linear relationship between the amount of mineral present and abrasiveness as a function of particle shape. 3.4. The effect of adding rounded quartz and angular pyrite of different sizes to the abrasiveness of the coal/mineral mixture Fig. 5 shows the values for the Abrasion Index of the coal/mineral mixtures with additions of the rounded quartz in the 100– 212 mm bracket, angular pyrite in the 50– 100 mm bracket, and a mixture of 50% rounded quartz (100 –212 mm) and 50% angular pyrite (50 – 100 mm). The dotted line is the calculated amount of abrasion the mixture of quartz and pyrite would have given based on the measurements of their individual contributions. This graph shows that the overall abrasion from two different mineral additions is not simply an additive function of the abrasion for each component.
J.J. Wells et al. / Fuel 83 (2004) 359–364
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This non-additive behaviour may be due to the rounded particles plastically deforming the metal surfaces, leading to locally work-hardened metal that could then reduce the cutting action of the angular particles.
the quartz and iron oxide to the coal without the rest of the slate (clays). The abrasion as a result of the other constituents in the slate makes up the difference between the two lines. The difference between the actual values for the abrasion with slate and the values calculated for abrasion from equivalent amounts of pure quartz and iron oxide in the slate is very small, especially considering the errors associated with determining the amounts of quartz and iron oxide in the slate. However, the slate does appear to have been more abrasive than the quartz and iron oxide contents alone would have suggested. Apatite, with a hardness similar to that of steel, is probably making a small contribution to the overall abrasiveness of the slate. It is impossible to determine if the aluminosilicate matrix is contributing to the abrasion or not, but if it is then it would be a very small contribution. It has been observed before that a softer material can abrade a hard material (e.g. the wear of diamond Atomic Force Microscopy tips on silicon wafers), but the effect is small [5]. In the case of a coal, any contribution from minerals below a hardness of 4.5 to the overall abrasiveness of the coal is likely to be negligible, except possibly for coals with very low quartz and pyrite contents.
3.5. The effect of slate on the abrasiveness of the coal/mineral mixture
4. Conclusions
Fig. 5. Abrasion Index versus vol% mineral addition for angular pyrite (50– 100 mm), rounded quartz (100–212 mm) and a 50:50 wt% mixture of the angular pyrite and rounded quartz. The dashed line represents the calculated Abrasion Index obtained by adding the individual abrasive values for the angular pyrite and rounded quartz for a 50:50 ratio.
The slate particles were found to be the least abrasive of the minerals added to the coal. This is as expected, given that the hardness of slates on the Mohs scale is , 3.5. However, there are components of the slate that are harder than steel, namely the quartz and iron oxides, which both have hardness values of 7. Effectively 15 wt% of the slate has a hardness greater than steel, and this material is generally in the 0 –50 mm size range. Fig. 6 shows a graph of the Abrasion Index for the coal/slate mixtures versus wt% addition of slate in the 100 –212 mm size range. The grey line shows the calculated Abrasion Index that might be expected for wear resulting from the equivalent of adding
Fig. 6. Abrasion Index versus wt% addition of the slate (100– 212 mm). The grey line is the calculated Abrasion Index for the cumulative effect of the quartz and iron oxide contents of the slate.
The effects of adding increasing amounts of different minerals to a washed Rossington coal on the Abrasion Index have been measured using a YGP test rig. Quartz and pyrite, with hardness values greater than that of mild steel, made significant contributions to the abrasiveness of the coal. Components of a slate with a lower hardness than the steel made very little contribution to the overall abrasiveness of the coal. Hence it is reasonable to assume that only those minerals in a coal that are harder than steel will significantly contribute to the abrasive nature of a coal. The abrasiveness of free angular quartz and pyrite was found to be similar when considered as a vol% addition. It is important to take into account the relative density of different minerals when considering abrasive wear. As the particle size of the quartz and pyrite increased up to a size of , 100 mm there was an increase in the value of the Abrasion Index for the coal/mineral mixtures. Above this size, values of the Abrasion Index appeared to be relatively independent of mineral size. This effect, known as the abrasion size effect, has been observed in other systems and is thought to be more significant for rounded, as opposed to angular, particles. Angular particles were far more abrasive than rounded particles. This is probably due to rounded particles causing plastic deformation of the metal surface, rather than cutting into the metal as an angular particle would do. The effect of adding both angular and rounded particles to the same batch of coal was not additive suggesting that the different abrasion mechanisms of angular and rounded particles are
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not mutually exclusive. Hence, for predictive purposes it may be necessary to determine the degree of angularity of hard minerals in a coal before grinding and milling.
also grateful to Powergen and Mitsui Babcock plc for their permission to publish this paper.
Acknowledgements
References
This work forms part of a DTI project funded under the ‘Cleaner Coal Research and Development’ programme (Project No. 218). The authors are grateful to Powergen for the use of their YGP Abrasion facilities and for preparing the coal according to British Standard BS 1016-111:1998, and to Mitsui Babcock plc for supplying the coal. The authors are
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